1. Field of the Invention
The present invention relates to a vehicle for towing a trailer and particularly relates to a movement stabilizing apparatus for a passenger car.
2. Description of Related Art
In automobiles (vehicles) towing trailers, a combination vehicle of tractor/trailer may begin to roll due to excessive speed, bad road conditions, crosswinds or the like. Such a problem arises particularly in towing trailers which do not have their own actuators and sensors for stabilizing the traveling dynamics. In particular, for trailers which have almost the same weights as passenger cars serving as tractors, the problem occurs. Thus, stability problems occur in the passenger cars which are towing travel trailers, for example.
If a snake movement or a pendulum movement occurs in a combination vehicle composed of an automobile (vehicle) and a trailer, the trailer oscillates about its vertical axis and also oscillates the passenger car serving as tractor via a trailer hitch. If the vehicular speed is equal to or smaller than a so-called critical speed, then the oscillations are damped. If the vehicular speed is equal to the critical speed, the oscillations are undamped. If the vehicular speed is more than the critical speed, the amplitude of the oscillations increases.
The value of the critical speed is a function of geometric data such as wheelbase and drawbar length, a function of the mass of the tractor (vehicle) and trailer, a function of the rotational inertia moment, and a function of a skid-angle rigidity of the axles. In a combination vehicle of the passenger car type, this value typically varies in a range from 90 to 130 km/h. The frequency of the snake movement or of the pendulum motion is approximately 0.5 to 1.5 Hz.
In order to solve such a problem, there is known a stabilizing method and apparatus for damping the pendulum motion as described in U.S. Pat. No. 6,523,911 B1. According to the method and apparatus, in order to stabilize the traveling condition of a vehicle, particularly, a passenger car towing a trailer, the vehicle monitors a lateral dynamics value such as a lateral acceleration or a yaw rate with respect to the pendulum motion to thereby detect the pendulum motion. When the pendulum motion is detected, the yaw moment which is almost periodical and almost opposite in the phase with respect to the pendulum motion is generated by the automatic braking operation and applied to the vehicle. In this manner, the snake movement of the combination vehicle composed of a tractor (vehicle) and a trailer can be avoided and the traveling condition of the combination vehicle can be stabilized.
In the aforesaid related art, it is determined that the pendulum movement occurs when a deviation ωe (ωe=ω−.ωt) between a yaw rate ω on the tractor (vehicle) side and a target yaw rate ωt exceeds a predetermined threshold value. Thus, a control value for damping the pendulum movement is determined so as to generate a yaw moment of which phase is in opposite to that of the deviation ωe. The target yaw rate ωt is determined by a mathematical model which is a function of a vehicular speed Vf and the steering angle δ of a front wheel.
However, when the target yaw rate ωt is used for detecting the pendulum movement, by the reasons explained below, the suitable control value for damping the pendulum movement and the suitable output timing (phase) thereof can not be determined and hence there may arises an inconvenience for the control.
(1) When an output value of a yaw rate sensor drifts due to the zero point deviation etc. of the output value of the yaw rate sensor or a steering angle sensor, there exists a steady deviation between the yaw rate to and the target yaw rate ωt. Thus, since the accurate cycle and amplitude of the pendulum movement can not be obtained, the suitable control value can not be calculated.
(2) Normally, the condition at the time of combining the trailer is not taken into consideration for the calculation of the target yaw rate ωt of a vehicle. Thus, when a vehicle coupled with a trailer turns, there arises the steady deviation between the actual yaw rate to and the target yaw rate ωt due to the coupling with the trailer, whereby the suitable control value can not be calculated like the case (1).
(3) There arises a time delay until the fluid pressure of the wheel brake increases actually after the pendulum movement is detected and an instruction signal for starting the stabilizing control. Thus, since it is impossible to generate a yaw moment which phase is completely in opposite to that of the deviation ωe, that is, the control is delayed in phase and the pendulum movement can not be damped sufficiently.